Are you using grid snap? Also, when you say extents, I am assuming you are referring to the inner handles on the collision shape and not the outer ones.
At any rate, if you are using grid snap, you can work around this by configuring the snap to half of whatever your texture is set at. In this case it should be something like x10.5 and y24.5. I don't think this is very good practice however, as there should be uniformity among your sprites and collision shapes to ensure that everything works harmoniously.
As to why this happens, I am not really sure. It's generally not a bad idea to try and have your pixel art set to a workable scale prior to putting it into the editor, at least based on my (very) limited experience.